Tim Marshall

Diplomatic Editor

There are many questions about the alleged use of chemical weapons by the Assad regime in Syria including one of the most puzzling - if it's true they were used, why were they used now?

Why attempt a very limited attack, resulting in no obvious military advantage?

Why use them knowing that you risk losing the diplomatic support of Russia and China, knowing you risk bringing in western intervention, and therefore risk losing the gains you've made militarily in the past few weeks?

Why now? It doesn't make sense.

If the evidence of use was overwhelming, the question would still remain. But the evidence is underwhelming.

The British Ministry of Defence is said to have obtained soil samples from inside Syria which have tested positive for sarin.

However, it is not known who had the soil samples before they were passed to the British, and the MoD will not publish its findings.

Given the debacle over the use of intelligence in the Iraq War, its reluctance to go public with information suggests it is not sure of its veracity but there is also the problem that publishing even limited evidence risks compromising sources inside Syria.

It's a similar story in the US where even the most hawkish member of the Senate, John McCain, agrees that the evidence "may not be airtight".

The White House has backtracked from statements late last week and now admits that "the chain of custody (of evidence) is not clear so we cannot confirm how the exposure occurred and under what conditions".

This translates as "we have no idea where this evidence came from or who had it, or even if someone planted it and passed it on to us".

On such a premise a case for war is not made.

The French have less information, but more clarity.

Foreign minister Laurent Fabius has said that Paris has "no evidence" that chemical weapons have been used in Syria.

They may have been, they may not, either way, the news surrounding the allegations has sparked a flurry of renewed debate about no fly zones, safe zones, and even Western intervention on the ground.

If it ever comes to the latter, it is almost certain that the first convoy into Syria will be bombed, as will the last one out, however many years and deaths of soldiers later.